1. Field of the Invention
The present invention relates to a fuel cell, and more particularly, to a fuel cell to which organic fuel is fed directly.
2. Description of the Related Art
Fuel cells are known as power generators that produce electrical energy through electrochemical reactions of fuel with oxygen or air. Since they are not based on the Carnot cycle applied to thermal power generation, their theoretical power generation efficiency is very high. Fuel cells can be used as power sources for small electrical/electronic devices, including portable devices, as well as for industrial, domestic, and transportation applications.
Fuel cells known so far can be classified into polymer electrolyte membrane (PEM) cells, phosphoric acid cells, molten carbonate cells, solid oxide cells, and other kinds depending on the type of electrolyte. The working temperature of fuel cells and constituent materials therefor are determined depending on the type of electrolyte used in a cell.
According to the way of supplying fuel to the anode, fuel cells can be classified into an external reformer type where fuel is supplied to the anode after being converted into hydrogen-rich gas by an external reformer and an internal reformer type or direct fuel supply type where fuel in gaseous or liquid state is directly supplied to the anode. Common fuels to be supplied directly to an anode of a fuel cell include natural gas and methanol. However, other hydrocarbon fuels and their derivatives may be supplied to the anode.
A representative example of direct liquid fuel cells is a direct methanol fuel cell (DMFC). DMFCs use aqueous methanol solution as fuel, and a proton exchange polymer membrane with ionic conductivity as an electrolyte. DMFCs do not require an external reformer, use fuel that is convenient to handle, and have the highest potential available as potable energy sources over other kinds of fuel cells.
Electrochemical reactions occurring in a DMFC are as follows: fuel is oxidized at the anode, and oxygen is reduced into water through a reaction with hydrogen ions at the cathode.Anode reaction: CH3OH+H2O→6H++6e−+CO2 Cathode Reaction: 1.5O2+6H++6e−→3H2OOverall Reaction: CH3OH+1.5O2→2H2O+CO2 
As is apparent from the above reaction schemes, methanol reacts with water at the anode to produce one carbon dioxide molecule, six hydrogen ions, and six electrons. The produced hydrogen ions migrate to the cathode through a polymer electrolyte membrane and react with oxygen and electrons, which are supplied via an external circuit, at the cathode to produce water. Summarizing the overall reaction in the DMFC, water and carbon dioxide are produced through the reaction of methanol with oxygen. As a result, a substantial part of the energy equivalent to the heat of combustion of methanol is converted into electrical energy.
The polymer electrolyte membrane with proton conductivity acts as a path for migrating the hydrogen ions, which are generated through the oxidation reaction at the anode, to the cathode and as a separator between the anode and the cathode. The polymer electrolyte membrane requires ionic conductivity that is high enough to rapidly migrate a large number of hydrogen ions, electrochemical stability, mechanical strength suitable for a separator, thermal stability at working temperature, ease of processing into a thin film so that its resistance to ionic conduction can be lowered, and anti-swelling property to liquid permeation.
As a common material for this polymer electrolyte membrane, a highly fluorinated polymer with sulfonate groups, such as Nafion (a registered trademark of Dupont), having a fluorinated alkylene backbone and a fluorinated vinyl ether side chain with sulfonate groups has been used. This kind of polymer electrolyte membrane consists of hydrophilic and hydrophobic groups and can contain an amount of water therein to provide good ionic conductivity.
Theoretically, methanol reacts with water in a 1:1 ratio by mole at the anode. Therefore, it is ideal to provide a 1:1 mixture of methanol and water by mole, for example, a 64% aqueous solution of methanol by weight. However, when such a high concentration of aqueous methanol solution is used as fuel, the unreacted methanol diffuses into and crosses over the hydrophilic polymer electrolyte membrane, thereby considerably reducing the performance of the fuel cell. To prevent cross-over of methanol, the amount of unreacted methanol must be reduced. Generally, in order to reduce the amount of unreacted methanol, a low concentration of aqueous methanol solution, for example, of 6-16% by weight is used as fuel. However, use of such a low-methanol solution inevitably leads to a lower working efficiency of the fuel cell. In addition, as the methanol permeates the polymer electrolyte membrane, a cathode catalyst is poisoned by the methanol, thereby the lifespan of the fuel cell decreases.
These problems are not limited only to methanol fuel cells and are common in fuel cells using other polar organic liquid fuels. Accordingly, many attempts have been made to prevent cross-over of polar organic liquid fuels, such as methanol and ethanol, in fuel cells. For example, U.S. Pat. Nos. 5,409,785; 5,795,668; 6,054,230; 6,242,122; 5,981,097; and 6,130,175 disclose multi-layered electrolyte membranes. U.S. Pat. Nos. 5,795,496; 6,510,047; and 6,194,474 disclose electrolyte membranes composed of heat resistant polymers. U.S. Pat. Nos. 5,919,583 and 5,849,428 disclose electrolyte membranes containing inorganic particles with proton conductivity. U.S. Pat. No. 4,985,315 discloses an electrolyte membrane containing amorphous materials having protonic conductivity. U.S. Pat. No. 5,672,439 discloses the use of an electrode having double catalyst layers.